I was just reading the posts about Sargent Johnson, and the one about Johnson's immunity being revoked intrigued me for a couple of reasons.

Spoilers!!

1. We know from Halo 3 that Johnson dies by the hands of Guilty Spark. Part of the reason for their getting rid of Johnson's immunity to flood could correspond with the flood DNA that was in his body. Without it, he can't regenerate, which enabled him to die.

2. Even if his flood DNA was still active, Guilty Spark's laser is basically a suped up Sentinel laser...which, we have all been told many, many times by guilty spark himself (evil library!) that the weapons are specifically calibrated to target and destroy flood. There goes Johnson's regeneration anyway.

3. There is a glitch where you can keep Johnson alive though the guilty spark encounter in Halo 3. (the cutscene counts him as there even though he's not, then he's still there when you leave). If Johnson is not protected, he can get infected and turn into a combat form (beware if you didn't take away the laser from him!). Here's a youtube link: http://www.youtube.com/watch?v=fA_tJrelaIE&feature=related. Of course, this doesn't happen in "normal" gameplay.

End Spoilers!!

Anyway, I found these facts interesting, but to build on the first reason of ascendant justice's post...ONI can't replicate Johnson's symptom even if they wanted to, as Master Chief destroyed the only copy of the evidence in First Strike. It was given to him to choose which report to give Command when he got back, and he chose to give them the one without the report on Johnson. And especially now that he's dead...there's no way they could use the information anyway.

Anyway, just some possible reasons why Bungie could have revoked his immunity.

There goes the only human being known to be infected by the Flood and make it through in one piece.